High-Hardness Steel Product and Method of Manufacturing the Same

ABSTRACT

Described is a hot-rolled steel strip product that includes a composition consisting of, in terms of weight percentages, 0.17% to 0.38% C, 0% to 0.5% Si, 0.1% to 0.4% Mn, 0.015% to 0.15% Al, 0.1% to 0.6% Cu, 0.2% to 0.8% Ni, 0.1% to 1% Cr, 0.01% to 0.3% Mo, 0% to 0.005% Nb, 0% to 0.05% Ti, 0% to 0.2% V, 0.0008% to 0.005% B, 0% to 0.025% P, 0.008% or less S, 0.01% or less N, 0% to 0.01% Ca, and the remainder being Fe and inevitable impurities, wherein the steel product has a Brinell hardness in the range of 420-580 HBW, and a corrosion index (ASTM G101-04) of at least 5.

FIELD OF INVENTION

The present invention relates to a high-hardness steel strip productexhibiting excellent resistance to climatic corrosion, a good balance ofhigh hardness and excellent mechanical properties such as impactstrength and bendability. The present invention further relates to amethod of manufacturing the high-hardness steel strip product.

BACKGROUND

High hardness has a direct effect on wear resistance of a steel product,the higher hardness the better wear resistance. By high hardness it ismeant that the Brinell hardness is at least 450 HBW and especially inthe range of 500 HBW to 650 HBW.

Wear resistant steels are also known as abrasion resistant steels. Theyare used in applications in which high resistance against abrasive andshock wear is required. Such applications can be found in e.g. miningand earth moving industry, and waste transportation. Wear resistantsteels are used for instance in gravel truck's bodies and excavatorbuckets, whereby longer service time of the vehicle components areachieved due to the high hardness provided by the wear resistant steels.The benefits of wear resistant steels are even more crucial when thepaint layer on a machine's outer surface is frequently exposed tomechanical stresses such as impacts which can cause scratch to paintlayers.

Such high hardness in steel product is typically obtained by martensiticmicrostructure produced by quench hardening steel alloy having highcontent of carbon (0.41-0.50 wt. %) after austenitization in thefurnace. In this process steel plates are first hot-rolled, slowlycooled to room temperature from the hot-rolling heat, reheated toaustenitization temperature, equalized and finally quench hardened. Thisprocess is hereinafter referred to as the reheating and quenching (RHQ)process. Examples of steels produced in this way are wear resistantsteels disclosed in CN102199737 or some commercial wear resistantsteels. Due to the relatively high content of carbon, which is requiredto achieve the desired hardness, the resulting martensite reactioncauses significant internal residual stresses to the steel. This isbecause the higher the carbon content the higher the lattice distortion.Therefore, this type of steel is very brittle and can even crack duringthe quench hardening. To overcome these drawbacks related tobrittleness, a tempering step after quench hardening is usuallyintroduced, which however increases the processing efforts and costs.

Due to the high carbon content these steels have deteriorated impactstrength, poor formability or bendability, and low resistance to stresscorrosion cracking (SCC). Stress corrosion cracking is the crackinginduced from the combined influence of tensile stress and a corrosiveenvironment. Usually, stress corrosion cracking starts as a pittingcorrosion with hard-to-detect fine cracks penetrating into the materialwhile most of the material surface appears intact. Stress corrosioncracking is classified as a catastrophic form of corrosion, as thedetection of such fine cracks can be very difficult and the damage noteasily predicted. There is a need of better approaches to decrease thecarbon content without compromising the hardness or any of the othermechanical properties, such as impact strength, formability/bendabilityor resistance to stress corrosion cracking.

CN102392186 and CN103820717 relate to RHQ steel plates having relativelylow carbon content (0.25-0.30 wt. % in CN102392186; 0.22-0.29 wt. % inCN103820717) and also relatively low manganese content. A tempering stepafter quench hardening is required for making such RHQ steel plates,which inevitably increases the processing efforts and costs.

EP2695960 relates to an abrasion-resistant steel product exhibitingexcellent resistance to stress corrosion cracking, which steel sheet canbe made in a process where direct quenching (DQ) may be performedimmediately after hot rolling, without the reheating treatment after hotrolling as in the RHQ process. The steel sheet of EP2695960 has arelatively low carbon content (0.20-0.30 wt. %) and a relatively highmanganese content (0.40-1.20 wt. %). In order to increase the resistanceto stress corrosion cracking, the base phase or main phase of themicrostructure of the steel product of EP2695960 must be made oftempered martensite. On the other hand, the area fraction of untemperedmartensite is restricted to 10% or less because the resistance to stresscorrosion cracking is reduced in the presence of untempered martensite.In balancing abrasion resistance and resistance to stress corrosioncracking, the steel product of EP2695960 has a surface hardness of 520HBW or less.

The present invention extends the utilization of the cost-effectivethermomechanically controlled processing (TMCP) in conjunction withdirect quenching (DQ) to produce a high-hardness steel strip productexhibiting improved resistance to climatic corrosion, guaranteed impactstrength values and excellent formability/bendability.

SUMMARY OF INVENTION

In view of the state of art, the object of the present invention is tosolve the problem of providing a high-hardness steel strip productexhibiting excellent resistance to climatic corrosion, guaranteed impactstrength values and excellent formability/bendability. The problem issolved by the combination of specific alloy designs with cost-efficientTMCP procedures which produces a metallographic microstructurecomprising mainly martensite.

In a first aspect, the present invention provides a hot-rolled steelstrip product comprising a composition consisting of, in terms of weightpercentages (wt. %):

C 0.17-0.38, preferably 0.21-0.35, more preferably 0.22-0.28 Si 0-0.5,preferably 0.01-0.5, more preferably 0.03-0.25 Mn 0.1-0.4, preferably0.15-0.3 Al 0.015-0.15 Cu 0.1-0.6, preferably 0.1-0.5, more preferably0.1-0.35 Ni 0-0.8, preferably 0.2-0.8 Cr 0.1-1, preferably 0.3-1, morepreferably 0.35-1, even more preferably 0.35-0.8 Mo 0.01-0.3, preferably0.03-0.3, more preferably 0.05-0.3 Nb    0-0.005 Ti 0-0.05, preferably0-0.035, more preferably 0-0.02 V 0-0.2, preferably 0-0.06 B0.0005-0.005, preferably 0.0008-0.005 P 0-0.025, preferably 0.001-0.025,more preferably 0.001-0.012 S 0-0.008, preferably 0-0.005 N 0-0.01,preferably 0-0.005, more preferably 0-0.004 Ca 0-0.01, preferably0-0.005, more preferably 0.0008-0.003remainder Fe and inevitable impurities.

Preferably, the aforementioned composition comprises, in terms of weightpercentages (wt. %):

Ti 0-0.005 N 0-0.003

Preferably, the aforementioned composition comprises, in terms of weightpercentages (wt. %):

Ti >0.005 and ≤0.05 N >0.003 and ≤0.01

Preferably, [Ni]>[Cu]/3, and more preferably [Ni]>[Cu]/2, wherein [Ni]is the amount of Ni in the composition, [Cu] is the amount of Cu in thecomposition.

The steel product is alloyed with the essential alloying elements Si,Cu, Ni and Cr, which provides good resistance against climatic corrosionand increases durability of a paint layer.

The steel product has a low content of Mn, which is important forimproving impact toughness and bendability.

The Ca/S ratio is adjusted such that CaS cannot form thereby improvingimpact toughness and bendability. The Ca/S ratio is preferably in therange of 1-2, more preferably 1.1-1.7, and even more preferably 1.2-1.6.

The level of Nb should be restricted to the lowest possible to increaseformability or bendability of the steel product. Elements such as Nb maybe present as residual contents that are not purposefully added.

The difference between residual contents and unavoidable impurities isthat residual contents are controlled quantities of alloying elements,which are not considered to be impurities. A residual content asnormally controlled by an industrial process does not have an essentialeffect upon the alloy.

In a second aspect, the present invention provides a method formanufacturing hot-rolled steel strip product comprising the followingsteps of

providing a steel slab consisting of the chemical composition asmentioned previously in the Summary and according to any one of theclaims 1 to 5;

heating the steel slab to the austenitizing temperature of 1200-1350°C.;

hot-rolling to the desired thickness at a temperature in the range ofAr₃ to 1300° C., wherein the finish rolling temperature is in the rangeof 800° C. to 960° C., preferably 870° C.-930° C., more preferably 885°C.-930° C.; and

direct quenching the hot-rolled steel strip product to a cooling end andcoiling temperature of 450° C. or less, preferably 250° C. or less, morepreferably 150° C. or less, and even more preferably 100° C. or less.

Optionally, a step of temper annealing is performed on the directquenched and coiled strip product at a temperature in the range of 150°C.-250° C. However, the step of temper annealing is not requiredaccording to the present invention.

The steel product is a steel strip having a thickness of 10 mm or less,preferably 8 mm or less, and more preferably 7 mm or less.

The obtained steel product has a microstructure comprising, in terms ofvolume percentages (vol. %), at least 90 vol. % martensite, preferablyat least 95 vol. % martensite, and more preferably at least 98 vol. %martensite, measured from ¼ thickness of the steel strip product. Themartensitic structure may be untempered, autotempered and/or tempered.Preferably, the martensitic structure is not tempered. More preferably,the aforementioned microstructure comprises more than 10 vol. %untempered martensite. Preferably, the microstructure comprises 0-1 vol.% residual austenite, and more preferably 0-0.5 vol. % residualaustenite. Typically, the microstructure also comprises bainite, ferriteand/or pearlite.

The obtained steel product has a prior austenite grain size of 50 μm orless, preferably 30 μm or less, more preferably 20 μm or less, measuredfrom ¼ thickness of the steel strip product.

The aspect ratio of a prior austenite grain structure is one of thefactors affecting a steel product's impact toughness and bendability. Inorder to improve impact toughness, the prior austenite grain structureshould have an aspect ratio of at least 1.5, preferably at least 2, andmore preferably at least 3. In order to improve bendability, the prioraustenite grain structure should have an aspect ratio of 7 or less,preferably 5 or less, and more preferably 1.5 or less. The obtainedsteel product according to the present invention has a prior austenitegrain structure with an aspect ratio in the range of 1.5-7, preferably1.5-5, and more preferably 2-5, which ensures that a good balance ofexcellent impact toughness and excellent bendability can be achieved.

The obtained steel product has a good balance of hardness and othermechanical properties such as improved resistance to climatic corrosionand excellent impact strength. The steel product has at least one of thefollowing mechanical properties: a Brinell hardness in the range of420-580 HBW, preferably 450-550 HBW, and more preferably 470-530 HBW;

a corrosion index (ASTM G101-04) ≥5, preferably 5.5, and more preferably≥6; a Charpy-V impact toughness of at least 34 J/cm² at a temperature of−20° C. or −40° C.

The steel product exhibits excellent bendability or formability. Thesteel product has a minimum bending radius of 3.4 t or less in ameasurement direction longitudinal to the rolling direction wherein thebending axis is longitudinal to rolling direction; a minimum bendingradius of 2.7 t or less in a measurement direction transversal to therolling direction wherein the bending axis is transversal to rollingdirection; and wherein t is the thickness of the steel strip product.

The steel product has a good balance of high hardness and excellentmechanical properties such as impact strength andformability/bendability. Consequently, the steel product exhibitsexcellent resistance to climatic corrosion.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates the microstructures.

DETAILED DESCRIPTION OF THE INVENTION

The term “steel” is defined as an iron alloy containing carbon (C).

The term climatic corrosion (a.k.a. atmospheric corrosion) refers tooutdoor corrosion caused by local environmental conditions.Environmental conditions are formed from weather phenomena like rain andsunshine. They are also affected by different impurities in the air likechlorides from sea water and sulfur compounds coming from volcanicactivity and industry or mining.

The term “Brinell hardness (HBW)” is a designation of hardness of steel.The Brinell hardness test is performed by pressing a 10 mm sphericaltungsten carbide ball against a clean prepared surface using a 3000kilogram force, producing an impression, measured and given a specialnumerical value.

The term “corrosion index (ASTM G101-04)” refers to the American Societyfor Testing and Materials (ASTM) standard G101 which is currently theonly available guide to quantify the atmospheric corrosion resistance ofweathering steels as a function of their composition.

The term “accelerated continuous cooling (ACC)” refers to a process ofaccelerated cooling at a cooling rate down to a temperature withoutinterruption.

The term “ultimate tensile strength (UTS, Rm)” refers to the limit, atwhich the steel fractures under tension, thus the maximum tensilestress.

The term “yield strength (YS, Rp_(0.2))” refers to 0.2% offset yieldstrength defined as the amount of stress that will result in a plasticstrain of 0.2%.

The term “total elongation (TEL)” refers to the percentage by which thematerial can be stretched before it breaks; a rough indicator offormability, usually expressed as a percentage over a fixed gauge lengthof the measuring extensometer. Two common gauge lengths are 50 mm (A₅₀)and 80 mm (A₈₀).

The term “minimum bending radius (Ri)” is used to refer to the minimumradius of bending that can be applied to a test sheet without occurrenceof cracks.

The term “bendability” refers to the ratio of Ri and the sheet thickness(t).

The alloying content of steel together with the processing parametersdetermines the microstructure which in turn determines the mechanicalproperties of the steel.

Alloy design is one of the first issues to be considered when developinga steel product with targeted mechanical properties. Next the chemicalcomposition according to the present invention is described in moredetails, wherein % of each component refers to weight percentage.

Carbon C is used in the range of 0.17% to 0.38%. C alloying increasesstrength of steel by solid solution strengthening, and hence C contentdetermines the strength level. C is used in the range of 0.17% to 0.38%depending on targeted hardness. If the carbon content is less than0.17%, it is difficult to achieve a Brinell hardness of more than 420HBW. However, C has detrimental effects on weldability, impacttoughness, formability or bendability, and resistance to stresscorrosion cracking. Therefore, C content is set to not more than 0.38%.

Preferably, C is used in the range of 0.21% to 0.35%, and morepreferably 0.22% to 0.28%.

Silicon Si is used in an amount of 0.5% or less. Si is added to thecomposition to facilitate formation of a protective oxide layer undercorrosive climate conditions, which provides good resistance againstclimatic corrosion and increases the durability of a paint layer that iseasily damaged or removed from machines surfaces due to wear. Si iseffective as a deoxidizing or killing agent that can remove oxygen fromthe melt during a steelmaking process. Si alloying enhances strength bysolid solution strengthening, and enhances hardness by increasingaustenite hardenability. Also the presence of Si can stabilize residualaustenite. However, silicon content of higher than 0.5% mayunnecessarily increase carbon equivalent (CE) value thereby weakeningthe weldability. Furthermore, surface quality may be deteriorated if Siis present in excess.

As previously mentioned, Si is an important alloying element forproviding sufficient hardness and good resistance to climatic corrosion,and for increasing durability of a paint layer. Preferably, Si is usedin the range of 0.01% to 0.5%, and more preferably 0.03% to 0.25%.

Manganese Mn is used in the range of 0.1% to 0.4%. Mn alloying lowersmartensite start temperature (Ms) and martensite finish temperature(Mf), which can suppress autotempering of martensite during quenching.Reduced autotempering of martensite leads to higher internal stressesthat enhance the risk for quench-induced cracking or distortion ofshape. Although a lower degree of autotempered martensiticmicrostructures is beneficial to higher hardness, its negative effectson impact strength should not be underestimated.

Mn alloying also enhances strength by solid solution strengthening, andenhances hardness by increasing austenite hardenability. However, if theMn content is too high, hardenability of the steel will increase at theexpense of impact toughness. Excessive Mn alloying may also lead to C-Mnsegregation and formation of MnS, which could induce formation ofinitiation sites for pitting corrosion and stress corrosion cracking.

Thus, Mn is used in an amount of at least 0.1% to ensure hardenability,but not more than 0.4% to avoid the harmful effects as described aboveand to ensure excellent mechanical properties such as impact strengthand bendability. Preferably, a low level of Mn is used in the range of0.15% to 0.3%.

Aluminum Al is used in the range of 0.015% to 0.15%. Al is effective asa deoxidizing or killing agent that can remove oxygen from the meltduring a steelmaking process. Al also removes N by forming stable AlNparticles and provides grain refinement, which is beneficial to hightoughness, especially at low temperatures. Also Al stabilizes residualaustenite. However, an excess of Al may increase non-metallic inclusionsthereby deteriorating cleanliness.

Copper Cu is used in the range of 0.1% to 0.6%. Cu is added to thecomposition to facilitate formation of a protective oxide layer undercorrosive climate conditions, which provides good resistance againstclimatic corrosion and increases the durability of a paint layer that iseasily damaged or removed from machines surfaces due to wear. Cu maypromote formation of low carbon bainitic structures, cause solidsolution strengthening and contribute to precipitation strengthening. Cumay also have beneficial effects of inhibiting stress corrosioncracking. When added in excessive amounts, Cu deteriorates fieldweldability and the heat affected zone (HAZ) toughness. Therefore, theupper limit of Cu is set to 0.6%.

As previously mentioned, Cu is an important alloying element forproviding sufficient hardness and good resistance to climatic corrosion,and for increasing durability of a paint layer. Preferably, Cu is usedin the range of 0.1% to 0.5%, and more preferably 0.1% to 0.35%.

Nickel Ni is used in in an amount of 0.8% or less. Ni is used to avoidquench induced cracking and also to improve low temperature toughness.Ni is an alloying element that improves austenite hardenability therebyincreasing strength with no or marginal loss of impact toughness and/orHAZ toughness. Ni also improves surface quality thereby preventingpitting corrosion, i.e. initiation site for stress corrosion cracking.Ni is added to the composition to facilitate formation of a protectiveoxide layer under corrosive climate conditions, which provides goodresistance against climatic corrosion and increases the durability of apaint layer that is easily damaged or removed from machines surfaces dueto wear. However, nickel contents of above 0.8% would increase alloyingcosts too much without significant technical improvement. An excess ofNi may produce high viscosity iron oxide scales which deterioratesurface quality of the steel product. Higher Ni contents also havenegative impacts on weldability due to increased CE value and crackingsensitivity coefficient.

As previously mentioned, Ni is an important alloying element forproviding sufficient hardness and good resistance to climatic corrosionwith no or marginal loss of impact toughness, and for increasingdurability of a paint layer. Ni is preferably used in the range of 0.2%to 0.8%.

Chromium Cr is used in the range of 0.1% to 1%. Cr is added to thecomposition to facilitate formation of a protective oxide layer undercorrosive climate conditions, which provides good resistance againstclimatic corrosion and increases the durability of a paint layer that iseasily damaged or removed from machines surfaces due to wear. Cralloying provides better resistance against pitting corrosion therebypreventing stress corrosion cracking at an early stage. As mid-strengthcarbide forming element Cr increases the strength of both the base steeland weld with marginal expense of impact toughness. Cr alloying alsoenhances strength and hardness by increasing austenite hardenability.However, if Cr is used in an amount above 1% the HAZ toughness as wellas field weldability may be adversely affected.

As previously mentioned, Cr is an important alloying element forproviding sufficient hardness and good resistance to climatic corrosionwith no or marginal loss of impact toughness, and for increasingdurability of a paint layer. Preferably, Cr is used in the range of 0.3%to 1%, more preferably 0.35% to 1%, and even more preferably 0.35% to0.8%.

Molybdenum Mo is used in the range of 0.01% to 0.3%. Mo alloyingimproves impact strength, low-temperature toughness and temperingresistance. The presence of Mo enhances strength and hardness byincreasing austenite hardenability. Mo can be added to the compositionto provide hardenability in place of Mn. In the case of B alloying, Mois usually required to ensure the effectiveness of B. However, Mo is notan economically acceptable alloying element. If Mo is used in an amountof above 0.3% toughness may be deteriorated thereby increasing the riskof brittleness. An excessive amount of Mo may also reduce the effect ofB. Furthermore, the inventors have noticed that Mo alloying retardsrecrystallization of austenite thereby increasing the aspect ratio of aprior austenite grain structure. Therefore, the level of Mo contentshould be carefully controlled to prevent excessive elongation of theprior austenite grains which may deteriorate bendability of the steelproduct.

Preferably, Mo is used in the range of 0.03% to 0.3%, and morepreferably 0.05% to 0.3%.

Niobium Nb is used in an amount of 0.005% or less. Nb forms carbides NbCand carbonitrides Nb(C,N). Nb is considered to be the major grainrefining element. Nb contributes to strengthening and toughening ofsteels. Yet, Nb addition should be limited to 0.005% since an excess ofNb deteriorates bendability, in particular when direct quenching isapplied and/or when Mo is present in the composition. Furthermore, Nbcan be harmful for HAZ toughness since Nb may promote the formation ofcoarse upper bainite structure by forming relatively unstable TiNbN orTiNb(C,N) precipitates. The level of Nb should be restricted to thelowest possible to increase formability or bendability of the steelproduct.

Titanium Ti is used in an amount of 0.05% or less. TiC precipitates areable to deeply trap a significant amount of hydrogen H, which decreasesthe H diffusivity in the materials and removes some of the detrimental Hfrom the microstructure to prevent stress corrosion cracking. Ti is alsoadded to bind free N that is harmful to toughness by forming stable TiNthat together with NbC can efficiently prevent austenite grain growth inthe reheating stage at high temperatures. TiN precipitates can furtherprevent grain coarsening in the HAZ during welding thereby improvingtoughness. TiN formation suppresses BN precipitation, thereby leaving Bfree to make its contribution to hardenability. For this purpose, theratio of Ti/N is at least 3.4. However, if Ti content is too high,coarsening of TiN and precipitation hardening due to TiC develop and thelow-temperature toughness may be deteriorated. Therefore, it isnecessary to restrict titanium so that it is less than 0.05%.

Preferably, Ti is used in an amount of 0.035% or less, and morepreferably 0.02% or less. If the steel product has a low nitrogencontent of 0.003% or less, it is unnecessary to add Ti to ensure theboron hardenability effect, and the Ti content can be as low as 0.005%or less. If the nitrogen content is more than 0.003% but no more than0.01%, the Ti content can be more than 0.005% but no more than 0.05%.

Vanadium V is used in an amount of 0.2% or less. V has substantially thesame but smaller effects as Nb. V₄C₃ precipitates are able to deeplytrap a significant amount of hydrogen H, which decreases the Hdiffusivity in the materials and removes some of the detrimental H fromthe microstructure to prevent HIC. V is a strong carbide and nitrideformer, but V(C,N) can also form and its solubility in austenite ishigher than that of Nb or Ti. Thus, V alloying has potential fordispersion and precipitation strengthening, because large quantities ofV are dissolved and available for precipitation in ferrite. However, anaddition of more than 0.2% V has negative effects on weldability andhardenability.

Preferably, V is used in an amount of 0.06% or less.

Boron B is used in the range of 0.0005% to 0.005%. B is awell-established microalloying element to increase hardenability. Themost effective B alloying would preferably require the presence of Ti inan amount of at least 3.42 N to prevent formation of BN. In the presenceof an amount of 0.003% or less nitrogen, the Ti content can be loweredto 0.005% or less, which is beneficial to low-temperature toughness.Hardenability deteriorates if the B content exceeds 0.005%.

Preferably, B is used in the range of 0.0008% to 0.005%.

Calcium Ca is used in an amount of 0.01% or less. Ca addition during asteelmaking process is for refining, deoxidation, desulphurization, andcontrol of shape, size and distribution of oxide and sulphideinclusions. Ca is usually added to improve subsequent coating. However,an excessive amount of Ca should be avoided to achieve clean steelthereby preventing the formation of calcium sulfide (CaS) or calciumoxide (CaO) or mixture of these (CaOS) that may deteriorate themechanical properties such as bendability and SCC resistance.

Preferably, Ca is used in an amount of 0.005% or less, and morepreferably 0.0008% to 0.003% to ensure excellent mechanical propertiessuch as impact strength and bendability.

The Ca/S ratio is adjusted such that CaS cannot form thereby improvingimpact toughness and bendability. The inventors have noticed that, ingeneral, during the steelmaking process the optimal Ca/S ratio is in therange of 1-2, preferably 1.1-1.7, and more preferably 1.2-1.6 for cleansteel.

Unavoidable impurities can be phosphor P, sulfur S, nitrogen N. Theircontent in terms of weight percentages (wt. %) is preferably defined asfollows:

P 0-0.025, preferably 0.001-0.025, more preferably 0.001-0.012 S0-0.008, preferably 0-0.005, more preferably 0-0.002 N 0-0.01,preferably 0-0.005, more preferably 0-0.004

Other inevitable impurities may be hydrogen H, oxygen O and rare earthmetals (REM) or the like. Their contents are limited in order to ensureexcellent mechanical properties, such as impact toughness.

Austenite to martensite transformation in steels depends largely on thefollowing factors: chemical composition and some processing parameters,mainly reheating temperature, cooling rate and cooling temperature. Withregard to chemical composition, some alloying elements have a greaterimpact than others while others have a negligible impact. Equationsdescribing austenite hardenability may be used to assess the impact ofdifferent alloying elements on martensite formation during cooling. Onesuch equation is presented below. From this equation we can see thatcarbon has the biggest impact, Mn, Mo and Cr have an intermediate impactwhile Si and Ni have a lesser impact. Furthermore, the equation showsthat any single element is not crucial for martensite formation and thatthe absence of one element may be compensated with the amount of otheralloying elements and processing parameters, such as e.g. cooling rate.

$D_{i} = {6 \times {\exp\left\lbrack {7.1 \times \left( {C + \frac{Mn}{{5.8}7} + \frac{Mo}{{3.1}3} + \frac{Cr}{{6.2}8} + \frac{Si}{18} + \frac{Ni}{15}} \right)} \right\rbrack}}$

The steel product with the targeted mechanical properties is produced ina process that determines a specific microstructure which in turndictates the mechanical properties of the steel product.

The first step is to provide a steel slab by means of, for instance aprocess of continuous casting, also known as strand casting.

In the reheating stage, the steel slab is heated to the austenitizingtemperature of 1200-1350° C., and thereafter subjected to a temperatureequalizing step that may take 30 to 150 minutes. The reheating andequalizing steps are important for controlling the austenite graingrowth. An increase in the heating temperature can cause dissolution andcoarsening of alloy precipitates, which may result in abnormal graingrowth.

The final steel product has a prior austenite grain size of 50 μm orless, preferably 30 μm or less, more preferably 20 μm or less, measuredfrom ¼ thickness of the steel strip product.

In the hot rolling stage the slab is hot rolled to the desired thicknessat a temperature in the range of Ar3 to 1300° C., wherein the finishrolling temperature (FRT) is in the range of 800° C. to 960° C.,preferably 870° C.-930° C., more preferably 885° C.-930° C.

The aspect ratio of a prior austenite grain structure is one of thefactors affecting a steel product's impact toughness and bendability. Inorder to improve impact toughness, the prior austenite grain structureshould have an aspect ratio of at least 1.5, preferably at least 2, andmore preferably at least 3. In order to improve bendability, the prioraustenite grain structure should have an aspect ratio of 7 or less,preferably 5 or less, and more preferably 1.5 or less. A desired aspectratio of prior austenite grains can be achieved by adjusting a number ofparameters such as finish rolling temperature, strain/deformation,strain rate, and/or alloying with the elements such as Mo that retardrecrystallization of austenite.

The obtained steel product according to the present invention has aprior austenite grain structure with an aspect ratio in the range of1.5-7, preferably 1.5-5, and more preferably 2-5, which ensures that agood balance of excellent impact toughness and excellent bendability canbe achieved.

The obtained steel strip product has a thickness of 10 mm or less,preferably 8 mm or less, more preferably 7 mm or less.

The hot-rolled steel strip product is direct quenched to a cooling endand coiling temperature of 450° C. or less, preferably 250° C. or less,more preferably 150° C. or less, and even more preferably 100° C. orless. The cooling rate is at least 30° C./s.

The direct quenched steel strip product is coiled at temperature of 450°C. or less, preferably 250° C. or less, more preferably 150° C. or less,and even more preferably 100° C. or less.

The obtained steel strip product has a microstructure comprising, interms of volume percentages (vol. %), at least 90 vol. % martensite,preferably at least 95 vol. % martensite, and more preferably at least98 vol. % martensite, measured from ¼ thickness of the steel stripproduct. The martensitic structure may be untempered, autotemperedand/or tempered. Preferably, the martensitic structure is not tempered.More preferably, the aforementioned microstructure comprises more than10 vol. % untempered martensite. Preferably, the microstructurecomprises 0-1 vol. % residual austenite, and more preferably 0-0.5 vol.% residual austenite. Typically, the microstructure also comprisesbainite, ferrite and/or pearlite.

Optionally, an extra step of temper annealing is performed at atemperature in the range of 150° C.-250° C.

The steel strip product has a good balance of hardness and othermechanical properties such as excellent impact strength, improvedresistance to climatic corrosion and excellent formability/bendability.

The steel strip product has a high Brinell hardness in the range of420-580 HBW, preferably 450-550 HBW, and more preferably 470-530 HBW.

The steel strip product has a corrosion index (ASTM G101-04) of at least5, preferably at least 5.5, and more preferably at least 6, whichindicates improved resistance against climatic corrosion. The durabilityof a paint layer is increased and the repainting interval can be 1.5-2times longer by using the steel product of the invention.

The corrosion index (ASTM G101-04) is used for estimating long termatmospheric corrosion of low alloy steels in various environments. Thecorrosion index (ASTM G101-04) equation is formed with a statisticalmethod from long term outdoor corrosion exposure tests, which equationis represented as follows.

I_(ASTMG101)=26.01(% Cu)+3.88(% Ni)+1.20(% Cr)+1.49(% Si)+17.28(%P)−7.29(% Cu)(% Ni)−9.10(% Ni)(% P)−33.39(% Cu)²

The steel strip product with high hardness has a Charpy-V impacttoughness of at least 34 J/cm² at a temperature of −20° C. or −40° C.thereby fulfilling the conventional impact strength requirements.

The steel strip product exhibits excellent bendability or formability.The steel product has a minimum bending radius of 3.4 t or less in ameasurement direction longitudinal to the rolling direction wherein thebending axis is longitudinal to rolling direction; a minimum bendingradius of 2.7 t or less in a measurement direction transversal to therolling direction wherein the bending axis is transversal to rollingdirection; and wherein t is the thickness of the steel strip product.

The following examples further describe and demonstrate embodimentswithin the scope of the present invention. The examples are given solelyfor the purpose of illustration and are not to be construed aslimitations of the present invention, as many variations thereof arepossible without departing from the scope of the invention.

The chemical compositions used for producing the tested steel stripproducts are presented in Table 1.

The manufacturing conditions for producing the tested steel stripproducts are presented in Table 2.

The mechanical properties of the tested steel strip products arepresented in Table 3.

Microstructure Microstructure can be characterized from SEM micrographsand the volume fraction can be determined using point counting or imageanalysis method. The microstructures of the tested inventive examplesno. 1-4 all have a main phase of at least 90 vol. % martensite. FIG. 1is an SEM image on the RD-ND plane from ¼ thickness of the steel stripno. 1, where the prior austenite grain boundaries are visualized. Theprior austenite grain structure of the steel strip no. 1 has an aspectratio of 3.4.

Brinell hardness HBW The Brinell hardness test is performed by pressinga 10 mm spherical tungsten carbide ball against a clean prepared surfaceusing a 3000 kilogram force, producing an impression, measured and givena special numerical value. The measurement is done perpendicular to theupper surface of the steel sheet at 10-15% depth from the steel surface.As shown in Table 3, each one of the inventive examples no. 1-4 exhibitsa Brinell harness in the range of 475-491 HBW. The comparative exampleno. 5 exhibits a Brinell harness of 486 HBW while the comparativeexample no. 6 exhibits a Brinell harness of 469 HBW.

Corrosion index (ASTM G101-04) The corrosion index (ASTM G101-04) iscalculated based on the American Society for Testing and Materials(ASTM) standard G101. As shown in Table 3, each one of the inventiveexamples no. 1-4 has a corrosion index (ASTM G101-04) of at least 5.28.On the other hand, the comparative examples no. 5 and 6 have a muchlower corrosion index (ASTM G101-04) of 3.4 and 1.04 respectively.

Charpy-V impact toughness The impact toughness values at −20° C. or −40°C. were obtained by Charpy V-notch tests according to the ASME (AmericanSociety of Mechanical Engineers) Standards. The inventive examples no. 1and 2 have a Charpy-V impact toughness of 63 J/cm² and 45 J/cm²respectively at a temperature of −20° C. (Table 3). Each one of theinventive examples no. 1-4 has a Charpy-V impact toughness in the rangeof 38-120 J/cm² at a temperature of −40° C. if the measurement directionis longitudinal to the rolling direction. Each one of the inventiveexamples no. 1-4 has a Charpy-V impact toughness in the range of 58-105J/cm² at a temperature of −40° C. if the measurement direction istransversal to the rolling direction. The impact toughness of theinventive examples no. 1-4 is improved compared to the comparativeexample no. 6. The comparative example no. 5 has a better Charpy-Vimpact toughness values than the inventive examples no. 1 and 2 at theexpense of bendability.

Elongation Elongation was determined according ASTM E8 standard usingtransverse specimens of a produced batch of 2000 ton of plates. The meanvalue of total elongation (A₅₀) of the inventive examples no. 1 and 2 is11.6 and 11.3 respectively (Table 3), which is better than thecomparative examples no. 5 and 6 having a mean A₅₀ value of 10.1 and 9.1respectively. The comparative examples no. 5 and 6 have better A₅₀values than the inventive examples no. 3 and 4 at the expense ofCharpy-V impact toughness.

Bendability The bend test consists of subjecting a test piece to plasticdeformation by three-point bending, with one single stroke, until aspecified angle 90° of the bend is reached after unloading. Theinspection and assessment of the bends is a continuous process duringthe whole test series. This is to be able to decide if the punch radius(R) should be increased, maintained or decreased. The limit ofbendability (R/t) for a material can be identified in a test series if aminimum of 3 m bending length, without any defects, is fulfilled withthe same punch radius (R) both longitudinally and transversally. Cracks,surface necking marks and flat bends (significant necking) areregistered as defects.

According to the bend tests, each one of the inventive examples no. 1-4has a minimum bending radius of 3.3 t or less in a measurement directionlongitudinal to the rolling direction; a minimum bending radius of 2.6 tor less in a measurement direction transversal to the rolling direction;and wherein t is the thickness of the steel strip product (Table 3). Thecomparative example no. 5 exhibits lower bendability with a minimumbending radius of 3.7 tin a measurement direction longitudinal to therolling direction and a minimum bending radius of 2.2 tin a measurementdirection transversal to the rolling direction.

Yield strength Yield strength was determined according ASTM E8 standardusing transverse specimens of a produced batch of 2000 ton of plates.Each one of the inventive examples no. 1-4 has a mean value of yieldstrength (Rp_(0.2)) in the range of 1302 MPa to 1399 MPa, measured inthe longitudinal direction (Table 3). The comparative examples no. 5 and6 have a mean value of yield strength (Rp_(0.2)) of 1262 MPa and 1338MPa respectively, measured in the longitudinal direction (Table 3).

Tensile strength Tensile strength was determined according ASTM E8standard using transverse specimens of a produced batch of 2000 ton ofplates. Each one of the inventive examples no. 1-4 has a mean value ofultimate tensile strength (Rm) in the range of 1509 MPa to 1566 MPa,measured in the longitudinal direction (Table 3). The comparativeexamples no. 5 and 6 have a mean value of ultimate tensile strength (Rm)of 1550 MPa and 1552 MPa respectively, measured in the longitudinaldirection (Table 3).

TABLE 1 Chemical compositions (wt. %). Steel type C Si Mn P S Al Cu NiCr A 0.251 0.098 0.246 0.008 0.0016 0.094 0.30 0.493 0.718 B 0.23 0.1790.200 0.007 −0.0006 0.051 0.16 0.51 0.39 C 0.233 0.179 0.714 0.0090.0006 0.035 0.009 0.506 0.713 D 0.262 0.175 1.19 0.008 0.0002 0.0480.01 0.035 0.212 Steel Ca N type Mo Nb Ti V B (ppm) (ppm) Remarks A0.098 0 0.016 0.04 0.0018 23 39 Inventive example B 0.05 0.001 0.0020.01 0.0011 8 24 Inventive example C 0.067 0 0.017 0.008 0.0017 21 31Comparative example D 0.005 0 0.015 0.008 0.0014 30 21 Comparativeexample

TABLE 2 Manufacturing conditions Temper Hot rolling annealing SteelStrip Heating Cooling Coiling Heating Holding strip Steel thicknesstemperature FRT rate temperature temperature time no. type (mm) (° C.)(° C.) (° C./s) (° C.) (° C.) (h) Remarks 1 A 6 1280 895 70 50 — —Inventive example 2 A 6 1280 925 70 50 — — Inventive example 3 B 6 1280900 — 50 200 8 Inventive example 4 B 3 1280 905 — 50 200 8 Inventiveexample 5 C 6 1280 870 55 50 — — Comparative example 6 D 6 1280 915 5550 — — Comparative example

TABLE 3 Mechanical properties Steel Rp_(0.2) Rm ChV ChV ChV Bendingstrip Steel Corr. (L) (L) (−20) T (−40) L (−40) T r/t no. type Index HBW(MPa) (MPa) A₅₀ (J/cm²) (J/cm²) (J/cm²) longit. transv. Remarks 1 A 6.74487 1399 1566 11.6 63 63 80 3.3 2.0 Inventive example 2 A 6.74 491 13371529 11.3 45 38 58 3.0 2.0 Inventive example 3 B 5.28 475 1355 1509 6.9— 120 83 2.3 1.3 Inventive example 4 B 5.28 487 1302 1549 8.8 — 120 1052.6 2.6 Inventive example 5 C 3.40 486 1262 1550 9.4 73 68 83 3.7 2.2Comparative example 6 D 1.04 469 1338 1552 10.0 32 30 42 — — Comparativeexample

1. A hot-rolled steel strip product comprising a composition consistingof, in terms of weight percentages (wt. %): C 0.17-0.38,  Si 0-0.5, Mn0.1-0.4,  Al 0.015-0.15    Cu 0.1-0.6,  Ni 0-0.8, Cr 0.1-1,    Mo0.01-0.3,   Nb  0-0.005 Ti  0-0.05, V 0-0.2, B 0.0005-0.005,   P 0-0.025, S  0-0.008, N  0-0.01, Ca  0-0.01,

remainder Fe and inevitable impurities, wherein the steel product has aBrinell hardness in the range of 420-580 HBW, and a corrosion index(ASTM G101-04) of at least
 5. 2. The steel product according to claim 1,wherein the amount of Ti is in the range of 0-0.005 wt. % when theamount of N is in the range of 0-0.003 wt. %.
 3. The steel productaccording to claim 1, wherein the amount of Ti is more than 0.005 wt. %and not more than 0.05 wt. % when the amount of N is more than 0.003 wt.% and not more than 0.01 wt. %.
 4. The steel product according to claim1, wherein [Ni]>[Cu]/3, and wherein [Ni] is the amount of Ni in thecomposition, [Cu] is the amount of Cu in the composition.
 5. The steelproduct according to claim 1, wherein the Ca/S ratio is in the range of1-2.
 6. The steel product according to claim 1, wherein the steelproduct has a Brinell hardness in the range of 450-550 HBW.
 7. The steelproduct according to claim 1, wherein the steel product has a corrosionindex (ASTM G101-04) of at least 5.5.
 8. The steel product according toclaim 1, wherein the steel product has a Charpy-V impact toughness of atleast 34 J/cm² at a temperature of −20° C. or −40° C. in transversaland/or longitudinal direction.
 9. The steel product according to claim1, wherein the steel product has a minimum bending radius of 3.4 t orless in a measurement direction longitudinal to the rolling direction; aminimum bending radius of 2.7 t or less in a measurement directiontransversal to the rolling direction; and wherein t is the thickness ofthe steel strip product.
 10. The steel product according to claim 1,wherein the steel product has a microstructure consisting of, in termsof volume percentages (vol. %), martensite ≥90, residual austenite 0-1,

remainder bainite, ferrite and/or pearlite.
 11. The steel productaccording to claim 1, wherein the steel product has a prior austenitegrain size of 50 μm or less.
 12. The steel product according to claim 1,wherein the steel product has a prior austenite grain structure with anaspect ratio in the range of 1.5-7.
 13. The steel product according toclaim 1, wherein the steel strip product has a thickness of 10 mm.
 14. Amethod for manufacturing the steel product according to claim 1comprising the following steps of providing a steel slab consisting ofthe chemical composition according to any one of the claims 1 to 5;heating the steel slab to the austenitizing temperature of 1200-1350°C.; hot-rolling to the desired thickness at a temperature in the rangeof Ar3 to 1300° C., wherein the finish rolling temperature is in therange of 800° C. to 960° C.; direct quenching the hot-rolled steel stripproduct to a cooling end and coiling temperature of 450° C. or less; andoptionally, temper annealing at a temperature in the range of 150°C.-250° C.